The present invention relates to a servo track writer capable of writing position information for the positioning control of a magnetic head to a magnetic disk having a metal or glass substrate and incorporated into a magnetic disk drive. The invention relates more particularly to a servo track writer to be applied to a magnetic disk drive for high-density recording.
The following patent documents are referred to below by ordinal number, and are hereby incorporated by reference:    1. Japanese Patent Laid-open No. 9-273316; and    2. Japanese Patent Laid-open No. 2002-15533.
The general magnetic disk drive comprises a head-disk assembly (HDA) for positioning a magnetic head held on the free end of a swing arm supported for turning on a pivot to read magnetic signals from and to write magnetic signals to a magnetic disk having a metal or glass substrate, and an electronic circuit board for controlling the HDA.
A servo pattern signal is recorded in the HDA. The servo pattern, i.e., a magnetic servo pattern, is used for positioning the magnetic head at a multiplicity of concentric tracks formed on the magnetic disk. The servo pattern signal used by recent magnetic disk drives are those of a sector servo system circumferentially intermittently arranged on the entire surfaces of a multiplicity of magnetic disks. According to the sector servo system, servo signals and information are recorded on the same track. Therefore, the magnetic head can accurately be positioned relative to the track. Thus, the sector servo system is suitable for recording data at a high density, increasing recording capacity, and is prevalently employed in general magnetic disk drives.
A description of method of recording a servo pattern signal of the conventional sector servo system follows. The HDA records a servo pattern signal by the magnetic head included therein. In recording the servo pattern signal, the arm holding the magnetic head on its free end is moved to position the magnetic head by using a sensor, such as a laser length-measuring device or an encoder, and the magnetic head thus positioned writes a servo pattern signal in the sector servo system to the magnetic disk. A special device for this purpose is called a servo track writer (hereinafter, abbreviated to “STW”).
The rotating speed of the magnetic disk is reduced to the lowest possible extent in writing a servo pattern signal by the conventional STW to write the servo pattern signal accurately. A prior art STW disclosed in, for example, Patent Document 1 writes a servo pattern signal while the magnetic disk is rotating at a speed that causes the magnetic head to float at a height not lower than a lower limit floating height determined from the magnetic head properties. According to Patent Document 1, the rotating speed of the magnetic disk for servo pattern signal writing is lower than a read/write rotating speed at which the magnetic disk is rotated for normal read/write operations. The written servo pattern signal is examined for verification by rotating the magnetic disk at the read/write rotating speed.
A prior art servo pattern signal writing method disclosed in, for example, Patent Document 2 controls the rotating speed of a spindle such that a rotating speed at which the spindle rotates while a servo pattern signal is written to a peripheral part of a magnetic disk is lower than that at which the spindle rotates while a servo pattern signal is written to a radially inner part near the center of the magnetic disk to minimize the difference between the relative speed of the peripheral part of the magnetic disk relative to the magnetic head and that of the radially inner part of the magnetic disk relative to the magnetic head.
[Problem to be Solved by the Invention]
As described above in Patent Document 1, the writing accuracy with respect to the radial direction of the magnetic disk of the STW that rotates the magnetic disk at a rotating speed lower than the read/write rotating speed in writing the servo pattern signal to the magnetic disk was satisfactory while the recording density of the magnetic disk was not very high. However, the level of writing accuracy of the STW has become unsatisfactory as the recording density of the magnetic disks increases year by year. Since the STW requires the magnetic disk to be rotated at a low rotating speed in writing the servo pattern signal to the magnetic disk, the STW takes much time to write the servo pattern signal to the magnetic disk. To complete writing the servo pattern signal in a fixed time, many STWs must be used and thereby the cost of equipment is increased.
The rotating speed at which the magnetic disk is rotated for writing the servo pattern signal to the magnetic disk by the STW (hereinafter, referred to as “STW rotating speed”) is determined taking into consideration technical conditions including the floating characteristic of the magnetic head, cost and time needed for the STW to write the servo pattern signal (hereinafter referred to as “STW time”), and the starting characteristic of the spindle motor supported in fluid bearings, to reduce the STW rotating speed with respect to the read/write rotating speed. As regards time required by the STW and the starting characteristic of the spindle motor, higher rotating speeds are advantageous regardless of the radial position of the magnetic head with respect to the magnetic head.
The relation between the floating characteristic of the magnetic head and the radial position of the magnetic head with respect to the magnetic disk is as follows. In conventional techniques including those disclosed in Patent Document 1, the floating characteristic of the magnetic head is a principal factor for determining the rotating speed of the magnetic disk for the operation of the STW. The servo pattern signal is written to the magnetic disk while the magnetic disk is rotated at a fixed, low rotating speed. For example, whereas the read/write rotating speed is 10,000 rpm, the STW rotating speed is 6,000 rpm. However, it has become difficult to position the magnetic head with satisfactory accuracy at a fixed STW rotating speed, such as 6,000 rpm, as head positioning accuracy (writing accuracy) became more precise, and it is necessary to decrease the fixed STW rotating speed to, for example, 5000 rpm, for servo pattern signal writing.
The floating height of the magnetic head has been decreased year by year to increase the areal recording density on the magnetic disk. Consequently, it has become difficult to decrease the rotating speed of the magnetic disk. The read/write rotating speed of the magnetic disk in the HDA has been increased to 10,000 rpm and 15,000 rpm.
Designing the STW is confronted by difficult conditions because the reduction of the rotating speed of the magnetic disk required by the STW, the reduction of the flying height of the magnetic head requiring increase in the rotating speed of the magnetic disk, and increase of the read/write rotating speed of the magnetic disk in the HDA are contradictory requirements.
The prior art technique mentioned in Patent Document 2 is intended for application to writing signals to a flexible disk. The flexible disk is rotated at a comparatively low rotating speed in the range of 2,400 to 3,600 rpm, and signals are written in a comparatively low areal recording density. Nothing is mentioned in Patent Document 2 about the effect of thermal drift due to the rotation of the spindle when the rotating speed of the spindle is high and the areal recording density is high.